College Chemistry : Atoms and Elements

Study concepts, example questions & explanations for College Chemistry

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Example Questions

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Example Question #1 : Isotopes

Isotopes differ in the number of __________.

Possible Answers:

lone pairs

neutrons

protons

covalent bonds

electrons

Correct answer:

neutrons

Explanation:

By definition, isotopes of a given element have the same number of protons and electrons, but differ in the number of neutrons. This causes a difference in the mass number (protons + neutrons) as well. Neither the number of protons nor the number of electrons changes with different isotopes of the same element.

Example Question #1 : Isotopes

Which of the following is not an isotope of hydrogen?

Possible Answers:

Tritium

Protium

Deuterium

Quatrium

Correct answer:

Quatrium

Explanation:

Isotopes are versions of an element with different numbers of neutrons. Hydrogen has three naturally occurring isotopes. , sometimes called protium, contains one electron, one proton, and no neutrons. , called deuterium, contains one electron, one proton, and one neutron. , called tritium, contains one electron, one proton, and two neutrons. Hydrogen has no such isotope that contains three neutrons.

Example Question #1 : Isotopes

Chromium has four naturally occurring isotopes.  has a mass of  and an abundance of  has a mass of  and an abundance of  has a mass of  and an abundance of  has a mass of  and an abundance of .

Using this information, estimate the atomic mass of chromium, in atomic mass units.

Possible Answers:

Correct answer:

Explanation:

In order to find the molar mass of an atom from its isotopes and their natural abundances, use the following equation:

 for all the given isotopes.

Since chromium has four isotopes, we will write the following equation to find its atomic mass:

Example Question #1 : Atoms And Elements

Isotopes of a given element differ in the number of __________.

Possible Answers:

Photons

Neutrons

Electrons

Protons

Correct answer:

Neutrons

Explanation:

Each element is defined by the number of protons its atoms contain. For example, hydrogen has one proton, helium has two protons, and lithium has three protons. Each element also has a characteristic number of neutrons. For example, hydrogen has zero neutrons, helium has two neutrons, and lithium has four neutrons.

Some elements, however, also have different "versions" of themselves: atoms which have a different number of neutrons, called isotopes. For example, there are three isotopes of hydrogen.  has one proton and zero neutrons.  has one proton and one neutron. Lastly,  has one proton and two neutrons. Carbon is another such element that has different isotopes. 

Example Question #1 : Radioactive Decay

What is the daughter nuclide when  undergoes alpha decay?

Possible Answers:

Correct answer:

Explanation:

Recall that when a particle undergoes alpha decay, the particle is emitting an alpha particle, which is the same as .

Now, write the following equation of the alpha decay:

Thus,  is the daughter nucleus.

Example Question #2 : Radioactive Decay

If  of a radioactive compound decays in  days, then what is the half-life of this radioactive compound?

Possible Answers:

Correct answer:

Explanation:

For this question, we're told that  of a radioactive compound decays in  days. We're asked to determine the half-life for this compound.

The first thing we need to do is write an expression for radioactive decay. Remembering that radioactive decay processes follow a first-order reaction rate, we can use the following expression.

We can further rewrite this expression as follows.

Also, recall that in the beginning we are starting with  of the compound. After  of it has decayed, we will be left with  of the compound at time . Furthermore, in order to solve for the half life, we will first need to find the rate constant, .

Now that we have the rate constant for the decay reaction, we're equipped with what we need in order to calculate the half-life.

To solve for the half-life, we can derive an expression using one already shown above.

Now, using the above expression, we can plug in the value for the rate constant that we calculated in order to solve for the half-life.

Example Question #3 : Radioactive Decay

What is the daughter nuclide when  undergoes beta decay?

Possible Answers:

Correct answer:

Explanation:

Recall that beta decay occurs when the nucleus of the parent atom emits an electron. We can then write the following equation to illustrate the beta decay of thorium.

Make sure that the atomic numbers and masses add up to the same on both sides of the equation.

Example Question #261 : College Chemistry

  undergoes three  decays to form which of these?

Possible Answers:

Correct answer:

Explanation:

In order to solve this problem we first must setup an equation representing the decay of  particles. We know that an  particle is equivalent to , so we can write our radioactive decay equation as:

where  is the unknown element

 is the atomic number

and  is the mass number. We can create two equations. One to solve for the mass number and one to solve for the atomic number.

Let's start with the atomic number:

Since we have an atomic number of  on the reactants side of the equation we can set that equal to  (the combined atomic number of three  particles + .

As an equation this is written as:

This simplifies to . This means that is the atomic number, and we can identify the element by looking at the periodic table for element number  as the atomic number identifies the element. The element is Platinum also written as .

Now we must solve for the mass number similarly:

Following the instructions above except for the mass number we create an equation with one unknown to solve for the mass number of  produced by this decay.:

therefore 

This means our final answer is .

This can be double-checked by plugging the mass number and atomic number into the decay equation and simplifying it.

Example Question #5 : Radioactive Decay

Which of the following sequences describe the decay process from  to ?

Possible Answers:

Alpha, alpha, beta

Beta, beta

Beta, beta, beta

Beta, gamma, alpha, alpha

Alpha, beta

Correct answer:

Beta, beta, beta

Explanation:

In order to determine the decay process that occurs we should check if the mass number charges or not in the decay. Since the mass number doesn't change we can eliminate any answer with   decay in it.

Now we can write out our decay equation. Since the atomic numbers aren't written in the question we must find them on the periodic table, where elements are ordered by their atomic number. In the case of Thallium, we find that it is element number  and in the case of Polonium we find that it is element .

So our equation is as follows:

where  is the element/particle name

 is the mass number

and  is the atomic number.

Since the mass number is the same on both sides of the equation we know that it is equal to .

Now we see that the atomic number increases by , which is the equivalent of a  on the product side of the equation. The only particle in the answer choices that is capable of having a negative atomic number value is a  particle, which is written as . In order to produce an atomic number change of  there must be   decays, which is the right answer.

Example Question #6 : Radioactive Decay

 undergoes a decay where neither the mass number, nor atomic number changes, what sort of decay is this?

Possible Answers:

Alpha ()

Electron capture

Beta ()

Positron emission

Gamma emission ()

Correct answer:

Alpha ()

Explanation:

Let's first look at the cases and determine which ones can maintain both the same atomic and mass number. We know that  decay produces a  nucleus so this cannot be the right answer as both the mass number and atomic number change with  decay.

As for  decay an electron is lost , so the atomic number increases, so therefore this cannot be the right answer.

In electron capture, an electron is gained so the atomic number decreases, so this cannot be the right answer either.

Positron emission means a positron  is lost, so this is not only equivalent to electron capture, but the atomic number changes by decreasing meaning this cannot be the right answer.

Therefore the right answer is gamma  decay, as in gamma decay the nuclei comes to a stable state and in order to do this must release ONLY energy, so neither the atomic number nor mass number change.

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